An important challenge faced by thermal protection system (“TPS”) designers is to provide reliable thermal protection for the crew or payload inside, within a limited fraction of the available vehicle mass. Generally, the design, manufacture, and implementation of the TPS are usually at the top of a reentry vehicle development risk list. This is especially the case when human occupants are involved with the reentry vehicle because the TPS is a sub-system that usually has no redundancy and a reentry vehicle is subjected to very high temperatures during atmospheric reentry. Specifically, the windward surface of a reentry vehicle is typically exposed to extreme temperatures and may be protected with ablative materials. In FIG. 1, a bottom perspective view of an example of an implementation of an atmospheric reentry vehicle 100 is shown. The atmospheric reentry vehicle 100 may include a vehicle structure 102 and a TPS shown as a heat shield 104 attached to the bottom of the vehicle structure 102. The heat shield 104 may be a separate structure that is attached to the bottom surface 200 of the vehicle structure 102 as shown in the exploded side view of FIG. 2.
The structure of a large reentry vehicle (or a hypersonic vehicle) is inherently flexible due to the imposed aero loads and other stresses. Unfortunately, this leads to a problem because existing ablator materials are stiff and have low strain to failure characteristics, resulting in cracking through-the-thickness of the ablator material during deflection.
Attempts at solving this problem have included utilizing structurally supporting scaffolds (i.e., a reinforcing core such as, for example, a honeycomb structure) to form an ablator having a reinforcing core (herein referred to as an “ARC”). An example of a core-reinforcement 300 is shown in FIGS. 3A and 3B.
In FIG. 3A, a partial top view of an example of an implementation of a core-reinforcement 300 is shown that may be utilized in the heat shield shown in FIGS. 1 and 2. In this example, the core-reinforcement 300 may include a plurality of cells 302 within the honeycomb structure of the core-reinforcement 300. In FIG. 3B, a partial side view of the core-reinforcement 300 is shown along a cutting plane A-A′ 304 looking into the side of the core-reinforcement 300. In this example, the height 306 of core-reinforcement 300 may be a few inches high, such as, for example, 2 to 4 inches in height 306. In this example, the cells 302 of the core-reinforcement 300 are filled with ablative material (not shown) and cured to form the ARC.
Generally, a method for making an ARC includes injecting (also referred to as “gunning”) the reinforcing core (such as a honeycomb structure) with an ablative material (such as, for example, the material described in United States (“U.S.”) U.S. Pat. No. 6,627,697, which is incorporated by reference herein in its entirety) to form the ARC. The ARC is then cured in an autoclave. The cured ARC may then be secured to the reentry vehicle to form an ARC-protected structure. This process prohibits bond verification, because of the large loads involved (i.e., the size and weight of the ARC) in this method would typically damage and/or destroy the underlying vehicle structure bottom surface 200. Additionally, other problems may include lifting, moving, or tipping a smaller vehicle with the bond verification loads. Generally, the bond verification loads on an adjacent structure may require large ground support equipment and tooling and hard points on the vehicle capable of withstanding the resulting reaction load.
Bond verification is significant because it directly correlates with the structural integrity of the entire TPS. Specifically, the structural integrity of the ARC (and thus the entire reentry vehicle), at least in part, is determined by how well the ARC is secured to the reentry vehicle, or at the very least, adds confidence that the reentry vehicle will retain its structural integrity during and after reentry into Earth's atmosphere. TPS bond verification is particularly critical to the safety of manned reentry vehicles due to the lack of redundancy of the TPS.
Another problem associated with known TPS manufacturing techniques is that they are very labor intensive and time consuming. As an example, the Apollo program of the National Aeronautics and Space Administration's (“NASA”) utilized a reinforcing core that consisted of a fiberglass honeycomb matrix that was fabricated by injecting an ablative material into the honeycomb matrix. In the Apollo material, the ablative material was an epoxy novolac resin with special additives and the ARC was known as “Avcoat 5026-39H.” Unfortunately, in fabrication, the empty honeycomb cells had to be individually gunned with the ablative material by a human operator using a heated caulk-gun-like tool. Additionally, because the Avcoat 5026-39H has firm putty consistency at room temp, each tube of Avcoat 5026-39H needed to be preheated before it could be pumped from the heated caulk-gun-like tool to fill each honeycomb cell.
As an example, in the Apollo program over 100,000 individual honeycomb cells in the reinforcing core had to be manually gunned and progressively cured. In the case of the Apollo program, the process of fabricating an ARC took about nine months. More recently, the Orion Multi-Purpose Crew Vehicle being built by NASA has a 5 meter in diameter ARC that has approximately 330,000 cells within the fiberglass-phenolic honeycomb matrix of the reinforcing core. Again, as in the Apollo program, each cell of the 330,000 must be manually gunned by a human operator using a caulk-gun-like tool with an ablative material that is again Avcoat 5026-39H material and then the ARC is heated to help cure the ablative material inside the honeycomb cells of the ARC.
As such, there is a need for a vehicle ablator system that addresses the shortcomings discussed above.